Semiconductor strip active optical device

Abstract

A guide structure of a strip semiconductor active optical device includes a core structure extending as far as coupling surfaces. It includes at least three high index layers having refractive indices increased relative to that of the surrounding media to increase in these layers the power density of light to be processed, for example amplitude modulated, by the device. The high index layers have compositions such that they can apply this processing in response to electrical excitation and thicknesses greater than those of quantum wells. They are separated by lower refractive index dilutant layers with greater thicknesses such that a single propagation mode of light is guided by the guide structure. This mode has a thickness suited to coupling to an external optical component whilst confining the majority of the power of the light within the thickness of the core structure to favor the processing.

There is claimed: 
     
       1. An electrically controllable strip-semiconductor active optical device comprising: a vertical succession of semiconductive layers forming a longitudinal guide structure for guiding a light undergoing a controlled processing in said device,   said guide structure comprising a strip-shaped core structure, disposed between a bottom and a top confinement layer of said device, thereby imposing a single propagation mode defining a spatial distribution of power fractions of said light as guided by said guide structure,   said core structure comprising a succession of high index layers, having a higher refractive index than said bottom and top confinement layers, and being mutually separated by a dilutant material having a lower refractive index than said high index layers, said high index layers having compositions enabling them to perform said controlled processing in response to a controlled density of charge carriers therein,   said bottom and top confinement layers having thicknesses and dopant concentrations of respectively a first and a second mutually opposite types of conductivity for exchanging said charge carriers with said high index layers so as to control said charge carrier density in response to an electrical control signal applied to said core structure between said bottom and top confinement layers,   said vertical succession of semiconductive layers further including a control layer, located on said top confinement layer, and having a dopant concentration of said second type of conductivity higher than said confinement layers, so that said control layer has an increased electrical conductivity and subjects a power fraction of said light therein to a parasitic absorption; and   means for applying said control signal to said core structure from said control layer, whereby the greater said thickness of said top confinement layer the lower an efficiency of said control signal,   wherein said succession of high index layers includes at least three said high index layers having thicknesses greater than that of quantum wells so that said efficiency of said control signal is kept high, while said power fraction of said light subjected to said parasitic absorption in said control layer is kept low.   
     
     
       2. Device according to claim 1 wherein a global structure including all the semiconductor layers of said wafer has a constant sequence of layers within said active segment and as far as said coupling surface. 
     
     
       3. Device according to claim 2 wherein said wafer further has along its length first and second coupling segments extending between said active segment and first and second coupling surfaces, respectively, at least one of said electrodes being formed only in said active segment. 
     
     
       4. Device according to claim 3 wherein the width of said strip decreases progressively in at least one coupling segment from said active segment as far as said coupling surface to increase the thickness of said guided mode at this surface. 
     
     
       5. Device according to claim 2 wherein said layer delimited at least partially in the transverse direction is said top confinement layer, said control layer and said top electrode being present only in the area of said strip to constitute an etched strip guide structure. 
     
     
       6. A strip semiconductor active optical device comprising an optical guide structure that includes a core structure, extending as far as a coupling surface of the device, and at least three high index layers having refractive indices increased relative to that of surrounding media in order to increase in said layers a power density of light to undergo processing in the device, said high index layers having compositions enabling said layers to perform said processing in response to electrical excitation, and having thicknesses greater than that of quantum wells, said surrounding media comprising dilutant layers having a lower refractive index and greater thicknesses than said high index layers, said high index layers being separated by said dilutant layers so that a single mode of propagation of said light is guided by said guide structure and has a thickness suited to coupling the light to an external optical component from said coupling surface, with the majority of the power of said light confined within the thickness of said core structure to favor said processing; said device further comprising a semiconductor wafer defining mutually perpendicular longitudinal, transverse and vertical directions and horizontal planes perpendicular to said vertical direction, lengths widths and thicknesses being defined in said longitudinal, transverse and vertical direction, respectively,   said wafer having horizontal top and bottom surfaces extending in the longitudinal direction between two end surfaces, at least one of said end surfaces constituting a coupling surface which an optical component external to the device must face in order to achieve coupling, between said external component and the device, of light to be processed by the device,   said wafer comprising horizontal layers having a continuous crystal structure and given thicknesses in succession in the vertical direction to constitute structures each including one layer or a plurality of adjacent layers, the compositions, the thicknesses and the order of succession of the layers of each structure constituting a sequence of layers of said structure,   said wafer further comprising in succession from said bottom surface:   a base structure having a first type of conductivity, at least an upper part of said structure being constituted by a bottom confinement layer transparent to the light to be processed by the device;   a core structure transparent to the light to be processed by the device and including a high index layer having a refractive index increased relative to that of the surrounding materials, said high index layer having a thickness greater than that of a quantum well and a composition conferring upon it an energy gap whereby it interacts with the light to be processed by the device, this interaction being conditioned by a charge carrier density within said high index layer which is therefore an active layer:   a top confinement layer transparent to the light to be processed by the device, said top confinement layer being formed on the core structure and constituting with said bottom confinement layer and said core structure a guide structure, at least one layer of said guide structure being delimited in the transverse direction over at least part of its thickness to form a strip, said strip having a given width and extending in the longitudinal direction, so that the light to be processed by the device is guided monomodally by the guide structure in this longitudinal direction in a single guided mode having a width related to that of said strip, said mode also having a given thickness, the value of said charge carrier density within the width of said strip controlling interaction of said high index layer with the light of this guided mode so that this value constitutes a control density; and   a control layer formed on the top confinement layer at least in an active segment occupying at least part of the length of said wafer, this control layer having a second type of conductivity opposite to the first type with an increased dopant concentration given it a high electrical conductivity to enable it to modify said control density by means of an electrical control signal applied between the base structure and this control layer, this increased dopant concentration causing this control layer to absorb the light to be processed by the device,   wherein the top confinement layer has a limited thickness so that the control signal can modify effectively the control density, the top confinement layer having a thickness sufficiently large relative to that of said guided mode to limit absorption of light in this mode in the control layer;   said device further comprising bottom and top electrodes respectively formed on said bottom and top surfaces in said active segment to apply said control signal between said base structure and said control layer,   wherein the core structure includes at least, in vertical succession from said bottom confinement layer to said top confinement layer;   a high index layer constituting a bottom high index layer;   a bottom dilutant layer having a lower refractive index than said bottom high index layer;   a high index layer constituting a median high index layer;   a top dilutant layer having a lower refractive index than said bottom and median high index layers; and   a high index layer constituting a top high index layer, the sequence of layers of the guide structure being such that said single guided mode constitutes a thick mode having a thickness suited to coupling to an external optical component said guide structure extending longitudinally without any change in its sequence of layers within said active segment and as far as said coupling surface;   wherein the sequence of layers of the guide structure gives this structure a confinement ratio exceeding 70%, the confinement ratio being the ratio of the power of said guided mode within the thickness of said core structure, to the total power of said mode.   
     
     
       7. A strip semiconductor active optical device comprising an optical guide structure: that includes a core structure, extending as far as a coupling surface of the device, and at least three high index layers having refractive indices increased relative to that of surrounding media in order to increase in said layers a power density of light to undergo processing in the device, said high index layers having compositions enabling said layers to perform said processing in response to electrical excitation, and having thicknesses greater than that of quantum wells, said surrounding media comprising dilutant layers having a lower refractive index and greater thicknesses than said high index layers, said high index layers being separated by said dilutant layers so that a single mode of propagation of said light is guided by said guide structure and has a thickness suited to coupling the light to an external optical component from said coupling surface, with the majority of the power of said light confined within the thickness of said core structure to favor said processing; said device further comprising a semiconductor wafer defining mutually perpendicular longitudinal, transverse and vertical directions and horizontal planes perpendicular to said vertical direction, lengths, widths and thicknesses being defined in said longitudinal, transverse and vertical direction, respectively,   said wafer having horizontal top and bottom surfaces extending in the longitudinal direction between two end surfaces, at least one of said end surfaces constituting a coupling surface which an optical component external to the device must face in order to achieve coupling, between said external component and the device, of light to be processed by the device,   said wafer comprising horizontal layers having a continuous crystal structure and given thicknesses in succession in the vertical direction to constitute structures each including one layer or a plurality of adjacent layers, the compositions, the thicknesses and the order of succession of the layers of each structure constituting a sequence of layers of said structure,   said wafer further comprising in succession from said bottom surface:   a base structure having a first type of conductivity, at least an upper part of said structure being constituted by a bottom confinement layer transparent to the light to be processed by the device;   a core structure transparent to the light to be processed by the device and including a high index layer having a refractive index increased relative to that of the surrounding materials, said high index layer having a thickness greater than that of a quantum well and a composition conferring upon it an energy gap whereby it interacts with the light to be processed by the device, this interaction being conditioned by a charge carrier density within said high index layer which is therefore an active layer;   a top confinement layer transparent to the light to be processed by the device, said top confinement layer being formed on the core structure and constituting with said bottom confinement layer and said core structure a guide structure, at least one layer of said guide structure being delimited in the transverse direction over at least part of its thickness to form a strip, said strip having a given width and extending in the longitudinal direction, so that the light to be processed by the device is guided monomodally by the guide structure in this longitudinal direction in a single guided mode having a width related to that of said strip, said mode also having a given thickness, the value of said charge carrier density within the width of said strip controlling interaction of said high index layer with the light of this guided mode so that this value constitutes a control density; and   a control layer formed on the top confinement layer at least in an active segment occupying at least part of the length of said wafer, this control layer having a second type of conductivity opposite to the first type with an increased dopant concentration given it a high electrical conductivity to enable it to modify said control density by means of an electrical control signal applied between the base structure and this control layer, this increased dopant concentration causing this control layer to absorb the light to be processed by the device,   wherein the top confinement layer has a limited thickness so that the control signal can modify effectively the control density, the top confinement layer having a thickness sufficiently large relative to that of said guided mode to limit absorption of light in this mode in the control layer;   said device further comprising bottom and top electrodes respectively formed on said bottom and top surfaces in said active segment to apply said control signal between said base structure and said control layer,   wherein the core structure includes at least, in vertical succession from said bottom confinement layer to said top confinement layer;   a high index layer constituting a bottom high index layer;.   a bottom dilutant layer having a lower refractive index than said bottom high index layer;   a high index layer constituting a median high index layer;   a top dilutant layer having a lower refractive index than said bottom and median high index layers; and   a high index layer constituting a top high index layer, the sequence of layers of the guide structure being such that said single guided mode constitutes a thick mode having a thickness suited to coupling to an external optical component, said guide structure extending longitudinally without any change in its sequence of layers within said active segment and as far as said coupling surface;   wherein the sequence of layers of the guide structure confers a thickness of between 1 μm and 2 μm on said guided mode.   
     
     
       8. Device according to claim 7 wherein said high index layers and said dilutant and confinement layers have a difference in refractive index greater than 0.15 and individual thicknesses greater than 30 nm so as to concentrate the light in these layers to favor said interaction, the individual thicknesses of these high index layers being less than 300 nm so that said guided mode is a single mode, a dilution ratio being defined by the ratio of the total thickness of the core structure to the total thickness of the high index layers only, this dilution ratio being greater than 2 to confer said thickness on the guided mode.